The invention relates to a linear output stage for charge-coupled circuits wherein an evaluator electrode of the output stage is connected to the input of an amplifier and wherein a switching transistor is provided whose gate terminal is supplied with a switching pulse train so that the switching transistor blocks during a read-out process and is switched conductive for a specific length of time between two read-out processes.
Charge-coupled circuits (CCD) are of significance for the processing of analogue signals since they can be used as analogue shift registers. Normal output circuits of charge-coupled elements are constructed with source followers wherein the non-linear characteristic of the MOS source follower which has an external load resistor is substantially linearized by means of a highly ohmic source resistor. An output circuit of this kind is described in the publication by D. F. Barbe "Imaging Devices Using The Charge Coupled Concept", Proceedings of the IEEE, Vol. 63, No. 1, January 1975, pages 38 to 67, incorporated herein by reference. A fundamental disadvantage of the described output circuit is that the evaluation of the signal is mainly carried out by means of a space charge capacitance which is voltage dependent since it is dependent upon the space charge depth.
The German AS No. 2 602 520 corresponding to U.S. Pat. No. 4,074,206 incorporated herein by reference discloses a linear output amplifier for charge-coupled elements wherein an amplifier is provided with a switching transistor and a load element, wherein the switching transistor and the load element are connected in series to one another at a point which represents the output of the amplifier. A gate terminal of the switching transistor represents the input of the amplifier. This input is connected to the CCD arrangement. The output of the amplifier is connected via an auxiliary transistor to the input of the amplifier. The gate terminal of the auxiliary transistor is connected to a pulse source which blocks the transistor during the read-out process and switches it conductive between the read-out processes.
In this known circuit arrangement, in order to prevent pulse train input couplings, two additional transistors must be provided by means of which the source and drain terminals of the auxiliary transistor can be disconnected. However, this results in tolerance problems. Although this known circuit arrangement offers the advantage that the described MOS amplifier is always operated in the linear range, a non-linearity, due to the voltage-dependent space charge capacitance of the signal output, continues to prevail.